1. Field of the Invention
The present invention relates to a focusing lens having a distributed index of refraction type lens, and more particularly to a focusing lens for a photographing lens.
2. Description of the Prior Art
Most of conventional focusing lenses are made of medium having a uniform index of refraction. For example, a photographing standard lens having F-number of 1.8.about.2 is usually comprised of six lenses of homogeneous medium.
If it can be composed of a smaller number of lenses, work required for making the lens is reduced, a lens holding mechanism is simplified and a chance of introduction of a manufacturing error such as eccentricity is reduced, and hence it is very advantageous. However, in the prior art design which uses the homogeneous medium, it is very difficult to reduce the number of lenses while keeping the optical performance.
Recently, the distributed index of refraction type lens has been introduced as a new optical system and many proposals have been made for the application of this lens to an array lens which is an erected unity-magnification focusing element, and a collimator lens. The application to a photographing lens is reported by Atkinson (Applied Optics, Vol. 21, No. 6, 1982). In this proposal, a standard lens having a half image angle of 21.8.degree. and F-member of 2 is constructed by two distributed index of refraction type lenses. The number of lenses used is significantly smaller than that of the conventional homogeneous lenses. However, the distributed index of refraction type lens is much more complex to manufacture than the homogeneous lens and the use of two or more such lenses is not desirable from the standpoint of cost and mass productivity.